Shorten nox session names

[dhermes]

Suggestions:

• nox -s "unit_tests(python_version='3.6')" becomes nox -s "unit(py='3.6')"
• nox -s "system_tests(python_version='3.6')" becomes nox -s "system(py='3.6')"

This is not important, but should be able to do in a few minutes.

[dhermes]

@lukesneeringer It seems we've lost our "good-first-bug" label?